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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="HOT-STANDBY"
>25.5. Hot Standby</A
></H1
><P
>    Hot Standby is the term used to describe the ability to connect to
    the server and run read-only queries while the server is in archive
    recovery or standby mode. This
    is useful both for replication purposes and for restoring a backup
    to a desired state with great precision.
    The term Hot Standby also refers to the ability of the server to move
    from recovery through to normal operation while users continue running
    queries and/or keep their connections open.
   </P
><P
>    Running queries in hot standby mode is similar to normal query operation,
    though there are several usage and administrative differences
    explained below.
   </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="HOT-STANDBY-USERS"
>25.5.1. User's Overview</A
></H2
><P
>    When the <A
HREF="runtime-config-replication.html#GUC-HOT-STANDBY"
>hot_standby</A
> parameter is set to true on a
    standby server, it will begin accepting connections once the recovery has
    brought the system to a consistent state.  All such connections are
    strictly read-only; not even temporary tables may be written.
   </P
><P
>    The data on the standby takes some time to arrive from the primary server
    so there will be a measurable delay between primary and standby. Running the
    same query nearly simultaneously on both primary and standby might therefore
    return differing results. We say that data on the standby is
    <I
CLASS="FIRSTTERM"
>eventually consistent</I
> with the primary.  Once the
    commit record for a transaction is replayed on the standby, the changes
    made by that transaction will be visible to any new snapshots taken on
    the standby.  Snapshots may be taken at the start of each query or at the
    start of each transaction, depending on the current transaction isolation
    level.  For more details, see <A
HREF="transaction-iso.html"
>Section 13.2</A
>.
   </P
><P
>    Transactions started during hot standby may issue the following commands:

    <P
></P
></P><UL
><LI
><P
>       Query access - <TT
CLASS="COMMAND"
>SELECT</TT
>, <TT
CLASS="COMMAND"
>COPY TO</TT
>
      </P
></LI
><LI
><P
>       Cursor commands - <TT
CLASS="COMMAND"
>DECLARE</TT
>, <TT
CLASS="COMMAND"
>FETCH</TT
>, <TT
CLASS="COMMAND"
>CLOSE</TT
>
      </P
></LI
><LI
><P
>       Parameters - <TT
CLASS="COMMAND"
>SHOW</TT
>, <TT
CLASS="COMMAND"
>SET</TT
>, <TT
CLASS="COMMAND"
>RESET</TT
>
      </P
></LI
><LI
><P
>       Transaction management commands
        <P
></P
></P><UL
><LI
><P
>           <TT
CLASS="COMMAND"
>BEGIN</TT
>, <TT
CLASS="COMMAND"
>END</TT
>, <TT
CLASS="COMMAND"
>ABORT</TT
>, <TT
CLASS="COMMAND"
>START TRANSACTION</TT
>
          </P
></LI
><LI
><P
>           <TT
CLASS="COMMAND"
>SAVEPOINT</TT
>, <TT
CLASS="COMMAND"
>RELEASE</TT
>, <TT
CLASS="COMMAND"
>ROLLBACK TO SAVEPOINT</TT
>
          </P
></LI
><LI
><P
>           <TT
CLASS="COMMAND"
>EXCEPTION</TT
> blocks and other internal subtransactions
          </P
></LI
></UL
><P>
      </P
></LI
><LI
><P
>       <TT
CLASS="COMMAND"
>LOCK TABLE</TT
>, though only when explicitly in one of these modes:
       <TT
CLASS="LITERAL"
>ACCESS SHARE</TT
>, <TT
CLASS="LITERAL"
>ROW SHARE</TT
> or <TT
CLASS="LITERAL"
>ROW EXCLUSIVE</TT
>.
      </P
></LI
><LI
><P
>       Plans and resources - <TT
CLASS="COMMAND"
>PREPARE</TT
>, <TT
CLASS="COMMAND"
>EXECUTE</TT
>,
       <TT
CLASS="COMMAND"
>DEALLOCATE</TT
>, <TT
CLASS="COMMAND"
>DISCARD</TT
>
      </P
></LI
><LI
><P
>       Plugins and extensions - <TT
CLASS="COMMAND"
>LOAD</TT
>
      </P
></LI
></UL
><P>
   </P
><P
>    Transactions started during hot standby will never be assigned a
    transaction ID and cannot write to the system write-ahead log.
    Therefore, the following actions will produce error messages:

    <P
></P
></P><UL
><LI
><P
>       Data Manipulation Language (DML) - <TT
CLASS="COMMAND"
>INSERT</TT
>,
       <TT
CLASS="COMMAND"
>UPDATE</TT
>, <TT
CLASS="COMMAND"
>DELETE</TT
>, <TT
CLASS="COMMAND"
>COPY FROM</TT
>,
       <TT
CLASS="COMMAND"
>TRUNCATE</TT
>.
       Note that there are no allowed actions that result in a trigger
       being executed during recovery.  This restriction applies even to
       temporary tables, because table rows cannot be read or written without
       assigning a transaction ID, which is currently not possible in a
       Hot Standby environment.
      </P
></LI
><LI
><P
>       Data Definition Language (DDL) - <TT
CLASS="COMMAND"
>CREATE</TT
>,
       <TT
CLASS="COMMAND"
>DROP</TT
>, <TT
CLASS="COMMAND"
>ALTER</TT
>, <TT
CLASS="COMMAND"
>COMMENT</TT
>.
       This restriction applies even to temporary tables, because carrying
       out these operations would require updating the system catalog tables.
      </P
></LI
><LI
><P
>       <TT
CLASS="COMMAND"
>SELECT ... FOR SHARE | UPDATE</TT
>, because row locks cannot be
       taken without updating the underlying data files.
      </P
></LI
><LI
><P
>       Rules on <TT
CLASS="COMMAND"
>SELECT</TT
> statements that generate DML commands.
      </P
></LI
><LI
><P
>       <TT
CLASS="COMMAND"
>LOCK</TT
> that explicitly requests a mode higher than <TT
CLASS="LITERAL"
>ROW EXCLUSIVE MODE</TT
>.
      </P
></LI
><LI
><P
>       <TT
CLASS="COMMAND"
>LOCK</TT
> in short default form, since it requests <TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE MODE</TT
>.
      </P
></LI
><LI
><P
>       Transaction management commands that explicitly set non-read-only state:
        <P
></P
></P><UL
><LI
><P
>            <TT
CLASS="COMMAND"
>BEGIN READ WRITE</TT
>,
            <TT
CLASS="COMMAND"
>START TRANSACTION READ WRITE</TT
>
          </P
></LI
><LI
><P
>            <TT
CLASS="COMMAND"
>SET TRANSACTION READ WRITE</TT
>,
            <TT
CLASS="COMMAND"
>SET SESSION CHARACTERISTICS AS TRANSACTION READ WRITE</TT
>
          </P
></LI
><LI
><P
>           <TT
CLASS="COMMAND"
>SET transaction_read_only = off</TT
>
          </P
></LI
></UL
><P>
      </P
></LI
><LI
><P
>       Two-phase commit commands - <TT
CLASS="COMMAND"
>PREPARE TRANSACTION</TT
>,
       <TT
CLASS="COMMAND"
>COMMIT PREPARED</TT
>, <TT
CLASS="COMMAND"
>ROLLBACK PREPARED</TT
>
       because even read-only transactions need to write WAL in the
       prepare phase (the first phase of two phase commit).
      </P
></LI
><LI
><P
>       Sequence updates - <CODE
CLASS="FUNCTION"
>nextval()</CODE
>, <CODE
CLASS="FUNCTION"
>setval()</CODE
>
      </P
></LI
><LI
><P
>       <TT
CLASS="COMMAND"
>LISTEN</TT
>, <TT
CLASS="COMMAND"
>UNLISTEN</TT
>, <TT
CLASS="COMMAND"
>NOTIFY</TT
>
      </P
></LI
></UL
><P>
   </P
><P
>    In normal operation, <SPAN
CLASS="QUOTE"
>"read-only"</SPAN
> transactions are allowed to
    update sequences and to use <TT
CLASS="COMMAND"
>LISTEN</TT
>, <TT
CLASS="COMMAND"
>UNLISTEN</TT
>, and
    <TT
CLASS="COMMAND"
>NOTIFY</TT
>, so Hot Standby sessions operate under slightly tighter
    restrictions than ordinary read-only sessions.  It is possible that some
    of these restrictions might be loosened in a future release.
   </P
><P
>    During hot standby, the parameter <TT
CLASS="VARNAME"
>transaction_read_only</TT
> is always
    true and may not be changed.  But as long as no attempt is made to modify
    the database, connections during hot standby will act much like any other
    database connection.  If failover or switchover occurs, the database will
    switch to normal processing mode.  Sessions will remain connected while the
    server changes mode.  Once hot standby finishes, it will be possible to
    initiate read-write transactions (even from a session begun during
    hot standby).
   </P
><P
>    Users will be able to tell whether their session is read-only by
    issuing <TT
CLASS="COMMAND"
>SHOW transaction_read_only</TT
>.  In addition, a set of
    functions (<A
HREF="functions-admin.html#FUNCTIONS-RECOVERY-INFO-TABLE"
>Table 9-57</A
>) allow users to
    access information about the standby server. These allow you to write
    programs that are aware of the current state of the database. These
    can be used to monitor the progress of recovery, or to allow you to
    write complex programs that restore the database to particular states.
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="HOT-STANDBY-CONFLICT"
>25.5.2. Handling Query Conflicts</A
></H2
><P
>    The primary and standby servers are in many ways loosely connected. Actions
    on the primary will have an effect on the standby. As a result, there is
    potential for negative interactions or conflicts between them. The easiest
    conflict to understand is performance: if a huge data load is taking place
    on the primary then this will generate a similar stream of WAL records on the
    standby, so standby queries may contend for system resources, such as I/O.
   </P
><P
>    There are also additional types of conflict that can occur with Hot Standby.
    These conflicts are <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>hard conflicts</I
></SPAN
> in the sense that queries
    might need to be canceled and, in some cases, sessions disconnected to resolve them.
    The user is provided with several ways to handle these
    conflicts. Conflict cases include:

      <P
></P
></P><UL
><LI
><P
>         Access Exclusive locks taken on the primary server, including both
         explicit <TT
CLASS="COMMAND"
>LOCK</TT
> commands and various <ACRONYM
CLASS="ACRONYM"
>DDL</ACRONYM
>
         actions, conflict with table accesses in standby queries.
        </P
></LI
><LI
><P
>         Dropping a tablespace on the primary conflicts with standby queries
         using that tablespace for temporary work files.
        </P
></LI
><LI
><P
>         Dropping a database on the primary conflicts with sessions connected
         to that database on the standby.
        </P
></LI
><LI
><P
>         Application of a vacuum cleanup record from WAL conflicts with
         standby transactions whose snapshots can still <SPAN
CLASS="QUOTE"
>"see"</SPAN
> any of
         the rows to be removed.
        </P
></LI
><LI
><P
>         Application of a vacuum cleanup record from WAL conflicts with
         queries accessing the target page on the standby, whether or not
         the data to be removed is visible.
        </P
></LI
></UL
><P>
   </P
><P
>    On the primary server, these cases simply result in waiting; and the
    user might choose to cancel either of the conflicting actions.  However,
    on the standby there is no choice: the WAL-logged action already occurred
    on the primary so the standby must not fail to apply it.  Furthermore,
    allowing WAL application to wait indefinitely may be very undesirable,
    because the standby's state will become increasingly far behind the
    primary's.  Therefore, a mechanism is provided to forcibly cancel standby
    queries that conflict with to-be-applied WAL records.
   </P
><P
>    An example of the problem situation is an administrator on the primary
    server running <TT
CLASS="COMMAND"
>DROP TABLE</TT
> on a table that is currently being
    queried on the standby server.  Clearly the standby query cannot continue
    if the <TT
CLASS="COMMAND"
>DROP TABLE</TT
> is applied on the standby. If this situation
    occurred on the primary, the <TT
CLASS="COMMAND"
>DROP TABLE</TT
> would wait until the
    other query had finished. But when <TT
CLASS="COMMAND"
>DROP TABLE</TT
> is run on the
    primary, the primary doesn't have information about what queries are
    running on the standby, so it will not wait for any such standby
    queries. The WAL change records come through to the standby while the
    standby query is still running, causing a conflict.  The standby server
    must either delay application of the WAL records (and everything after
    them, too) or else cancel the conflicting query so that the <TT
CLASS="COMMAND"
>DROP
    TABLE</TT
> can be applied.
   </P
><P
>    When a conflicting query is short, it's typically desirable to allow it to
    complete by delaying WAL application for a little bit; but a long delay in
    WAL application is usually not desirable.  So the cancel mechanism has
    parameters, <A
HREF="runtime-config-replication.html#GUC-MAX-STANDBY-ARCHIVE-DELAY"
>max_standby_archive_delay</A
> and <A
HREF="runtime-config-replication.html#GUC-MAX-STANDBY-STREAMING-DELAY"
>max_standby_streaming_delay</A
>, that define the maximum
    allowed delay in WAL application.  Conflicting queries will be canceled
    once it has taken longer than the relevant delay setting to apply any
    newly-received WAL data.  There are two parameters so that different delay
    values can be specified for the case of reading WAL data from an archive
    (i.e., initial recovery from a base backup or <SPAN
CLASS="QUOTE"
>"catching up"</SPAN
> a
    standby server that has fallen far behind) versus reading WAL data via
    streaming replication.
   </P
><P
>    In a standby server that exists primarily for high availability, it's
    best to set the delay parameters relatively short, so that the server
    cannot fall far behind the primary due to delays caused by standby
    queries.  However, if the standby server is meant for executing
    long-running queries, then a high or even infinite delay value may be
    preferable.  Keep in mind however that a long-running query could
    cause other sessions on the standby server to not see recent changes
    on the primary, if it delays application of WAL records.
   </P
><P
>    Once the delay specified by <TT
CLASS="VARNAME"
>max_standby_archive_delay</TT
> or
    <TT
CLASS="VARNAME"
>max_standby_streaming_delay</TT
> has been exceeded, conflicting
    queries will be canceled.  This usually results just in a cancellation
    error, although in the case of replaying a <TT
CLASS="COMMAND"
>DROP DATABASE</TT
>
    the entire conflicting session will be terminated.  Also, if the conflict
    is over a lock held by an idle transaction, the conflicting session is
    terminated (this behavior might change in the future).
   </P
><P
>    Canceled queries may be retried immediately (after beginning a new
    transaction, of course).  Since query cancellation depends on
    the nature of the WAL records being replayed, a query that was
    canceled may well succeed if it is executed again.
   </P
><P
>    Keep in mind that the delay parameters are compared to the elapsed time
    since the WAL data was received by the standby server.  Thus, the grace
    period allowed to any one query on the standby is never more than the
    delay parameter, and could be considerably less if the standby has already
    fallen behind as a result of waiting for previous queries to complete, or
    as a result of being unable to keep up with a heavy update load.
   </P
><P
>    The most common reason for conflict between standby queries and WAL replay
    is <SPAN
CLASS="QUOTE"
>"early cleanup"</SPAN
>.  Normally, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> allows
    cleanup of old row versions when there are no transactions that need to
    see them to ensure correct visibility of data according to MVCC rules.
    However, this rule can only be applied for transactions executing on the
    master.  So it is possible that cleanup on the master will remove row
    versions that are still visible to a transaction on the standby.
   </P
><P
>    Experienced users should note that both row version cleanup and row version
    freezing will potentially conflict with standby queries. Running a manual
    <TT
CLASS="COMMAND"
>VACUUM FREEZE</TT
> is likely to cause conflicts even on tables with
    no updated or deleted rows.
   </P
><P
>    Users should be clear that tables that are regularly and heavily updated
    on the primary server will quickly cause cancellation of longer running
    queries on the standby. In such cases the setting of a finite value for
    <TT
CLASS="VARNAME"
>max_standby_archive_delay</TT
> or
    <TT
CLASS="VARNAME"
>max_standby_streaming_delay</TT
> can be considered similar to
    setting <TT
CLASS="VARNAME"
>statement_timeout</TT
>.
   </P
><P
>    Remedial possibilities exist if the number of standby-query cancellations
    is found to be unacceptable.  The first option is to set the parameter
    <TT
CLASS="VARNAME"
>hot_standby_feedback</TT
>, which prevents <TT
CLASS="COMMAND"
>VACUUM</TT
> from
    removing recently-dead rows and so cleanup conflicts do not occur.
    If you do this, you
    should note that this will delay cleanup of dead rows on the primary,
    which may result in undesirable table bloat. However, the cleanup
    situation will be no worse than if the standby queries were running
    directly on the primary server, and you are still getting the benefit of
    off-loading execution onto the standby.
    <TT
CLASS="VARNAME"
>max_standby_archive_delay</TT
> must be kept large in this case,
    because delayed WAL files might already contain entries that conflict with
    the desired standby queries.
   </P
><P
>    Another option is to increase <A
HREF="runtime-config-replication.html#GUC-VACUUM-DEFER-CLEANUP-AGE"
>vacuum_defer_cleanup_age</A
>
    on the primary server, so that dead rows will not be cleaned up as quickly
    as they normally would be.  This will allow more time for queries to
    execute before they are canceled on the standby, without having to set
    a high <TT
CLASS="VARNAME"
>max_standby_streaming_delay</TT
>.  However it is
    difficult to guarantee any specific execution-time window with this
    approach, since <TT
CLASS="VARNAME"
>vacuum_defer_cleanup_age</TT
> is measured in
    transactions executed on the primary server.
   </P
><P
>    The number of query cancels and the reason for them can be viewed using
    the <TT
CLASS="STRUCTNAME"
>pg_stat_database_conflicts</TT
> system view on the standby
    server. The <TT
CLASS="STRUCTNAME"
>pg_stat_database</TT
> system view also contains
    summary information.
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="HOT-STANDBY-ADMIN"
>25.5.3. Administrator's Overview</A
></H2
><P
>    If <TT
CLASS="VARNAME"
>hot_standby</TT
> is turned <TT
CLASS="LITERAL"
>on</TT
> in
    <TT
CLASS="FILENAME"
>postgresql.conf</TT
> and there is a <TT
CLASS="FILENAME"
>recovery.conf</TT
>
    file present, the server will run in Hot Standby mode.
    However, it may take some time for Hot Standby connections to be allowed,
    because the server will not accept connections until it has completed
    sufficient recovery to provide a consistent state against which queries
    can run.  During this period,
    clients that attempt to connect will be refused with an error message.
    To confirm the server has come up, either loop trying to connect from
    the application, or look for these messages in the server logs:

</P><PRE
CLASS="PROGRAMLISTING"
>LOG:  entering standby mode

... then some time later ...

LOG:  consistent recovery state reached
LOG:  database system is ready to accept read only connections</PRE
><P>

    Consistency information is recorded once per checkpoint on the primary.
    It is not possible to enable hot standby when reading WAL
    written during a period when <TT
CLASS="VARNAME"
>wal_level</TT
> was not set to
    <TT
CLASS="LITERAL"
>hot_standby</TT
> on the primary.  Reaching a consistent state can
    also be delayed in the presence of both of these conditions:

      <P
></P
></P><UL
><LI
><P
>         A write transaction has more than 64 subtransactions
        </P
></LI
><LI
><P
>         Very long-lived write transactions
        </P
></LI
></UL
><P>

    If you are running file-based log shipping ("warm standby"), you might need
    to wait until the next WAL file arrives, which could be as long as the
    <TT
CLASS="VARNAME"
>archive_timeout</TT
> setting on the primary.
   </P
><P
>    The setting of some parameters on the standby will need reconfiguration
    if they have been changed on the primary. For these parameters,
    the value on the standby must
    be equal to or greater than the value on the primary. If these parameters
    are not set high enough then the standby will refuse to start.
    Higher values can then be supplied and the server
    restarted to begin recovery again.  These parameters are:

      <P
></P
></P><UL
><LI
><P
>         <TT
CLASS="VARNAME"
>max_connections</TT
>
        </P
></LI
><LI
><P
>         <TT
CLASS="VARNAME"
>max_prepared_transactions</TT
>
        </P
></LI
><LI
><P
>         <TT
CLASS="VARNAME"
>max_locks_per_transaction</TT
>
        </P
></LI
></UL
><P>
   </P
><P
>    It is important that the administrator select appropriate settings for
    <A
HREF="runtime-config-replication.html#GUC-MAX-STANDBY-ARCHIVE-DELAY"
>max_standby_archive_delay</A
> and <A
HREF="runtime-config-replication.html#GUC-MAX-STANDBY-STREAMING-DELAY"
>max_standby_streaming_delay</A
>.  The best choices vary
    depending on business priorities.  For example if the server is primarily
    tasked as a High Availability server, then you will want low delay
    settings, perhaps even zero, though that is a very aggressive setting. If
    the standby server is tasked as an additional server for decision support
    queries then it might be acceptable to set the maximum delay values to
    many hours, or even -1 which means wait forever for queries to complete.
   </P
><P
>    Transaction status "hint bits" written on the primary are not WAL-logged,
    so data on the standby will likely re-write the hints again on the standby.
    Thus, the standby server will still perform disk writes even though
    all users are read-only; no changes occur to the data values
    themselves.  Users will still write large sort temporary files and
    re-generate relcache info files, so no part of the database
    is truly read-only during hot standby mode.
    Note also that writes to remote databases using
    <SPAN
CLASS="APPLICATION"
>dblink</SPAN
> module, and other operations outside the
    database using PL functions will still be possible, even though the
    transaction is read-only locally.
   </P
><P
>    The following types of administration commands are not accepted
    during recovery mode:

      <P
></P
></P><UL
><LI
><P
>         Data Definition Language (DDL) - e.g. <TT
CLASS="COMMAND"
>CREATE INDEX</TT
>
        </P
></LI
><LI
><P
>         Privilege and Ownership - <TT
CLASS="COMMAND"
>GRANT</TT
>, <TT
CLASS="COMMAND"
>REVOKE</TT
>,
         <TT
CLASS="COMMAND"
>REASSIGN</TT
>
        </P
></LI
><LI
><P
>         Maintenance commands - <TT
CLASS="COMMAND"
>ANALYZE</TT
>, <TT
CLASS="COMMAND"
>VACUUM</TT
>,
         <TT
CLASS="COMMAND"
>CLUSTER</TT
>, <TT
CLASS="COMMAND"
>REINDEX</TT
>
        </P
></LI
></UL
><P>
   </P
><P
>    Again, note that some of these commands are actually allowed during
    "read only" mode transactions on the primary.
   </P
><P
>    As a result, you cannot create additional indexes that exist solely
    on the standby, nor statistics that exist solely on the standby.
    If these administration commands are needed, they should be executed
    on the primary, and eventually those changes will propagate to the
    standby.
   </P
><P
>    <CODE
CLASS="FUNCTION"
>pg_cancel_backend()</CODE
> will work on user backends, but not the
    Startup process, which performs recovery. <TT
CLASS="STRUCTNAME"
>pg_stat_activity</TT
> does not
    show an entry for the Startup process, nor do recovering transactions
    show as active. As a result, <TT
CLASS="STRUCTNAME"
>pg_prepared_xacts</TT
> is always empty during
    recovery. If you wish to resolve in-doubt prepared transactions,
    view <TT
CLASS="LITERAL"
>pg_prepared_xacts</TT
> on the primary and issue commands to
    resolve transactions there.
   </P
><P
>    <TT
CLASS="STRUCTNAME"
>pg_locks</TT
> will show locks held by backends,
    as normal. <TT
CLASS="STRUCTNAME"
>pg_locks</TT
> also shows
    a virtual transaction managed by the Startup process that owns all
    <TT
CLASS="LITERAL"
>AccessExclusiveLocks</TT
> held by transactions being replayed by recovery.
    Note that the Startup process does not acquire locks to
    make database changes, and thus locks other than <TT
CLASS="LITERAL"
>AccessExclusiveLocks</TT
>
    do not show in <TT
CLASS="STRUCTNAME"
>pg_locks</TT
> for the Startup
    process; they are just presumed to exist.
   </P
><P
>    The <SPAN
CLASS="PRODUCTNAME"
>Nagios</SPAN
> plugin <SPAN
CLASS="PRODUCTNAME"
>check_pgsql</SPAN
> will
    work, because the simple information it checks for exists.
    The <SPAN
CLASS="PRODUCTNAME"
>check_postgres</SPAN
> monitoring script will also work,
    though some reported values could give different or confusing results.
    For example, last vacuum time will not be maintained, since no
    vacuum occurs on the standby.  Vacuums running on the primary
    do still send their changes to the standby.
   </P
><P
>    WAL file control commands will not work during recovery,
    e.g. <CODE
CLASS="FUNCTION"
>pg_start_backup</CODE
>, <CODE
CLASS="FUNCTION"
>pg_switch_xlog</CODE
> etc.
   </P
><P
>    Dynamically loadable modules work, including <TT
CLASS="STRUCTNAME"
>pg_stat_statements</TT
>.
   </P
><P
>    Advisory locks work normally in recovery, including deadlock detection.
    Note that advisory locks are never WAL logged, so it is impossible for
    an advisory lock on either the primary or the standby to conflict with WAL
    replay. Nor is it possible to acquire an advisory lock on the primary
    and have it initiate a similar advisory lock on the standby. Advisory
    locks relate only to the server on which they are acquired.
   </P
><P
>    Trigger-based replication systems such as <SPAN
CLASS="PRODUCTNAME"
>Slony</SPAN
>,
    <SPAN
CLASS="PRODUCTNAME"
>Londiste</SPAN
> and <SPAN
CLASS="PRODUCTNAME"
>Bucardo</SPAN
> won't run on the
    standby at all, though they will run happily on the primary server as
    long as the changes are not sent to standby servers to be applied.
    WAL replay is not trigger-based so you cannot relay from the
    standby to any system that requires additional database writes or
    relies on the use of triggers.
   </P
><P
>    New OIDs cannot be assigned, though some <ACRONYM
CLASS="ACRONYM"
>UUID</ACRONYM
> generators may still
    work as long as they do not rely on writing new status to the database.
   </P
><P
>    Currently, temporary table creation is not allowed during read only
    transactions, so in some cases existing scripts will not run correctly.
    This restriction might be relaxed in a later release. This is
    both a SQL Standard compliance issue and a technical issue.
   </P
><P
>    <TT
CLASS="COMMAND"
>DROP TABLESPACE</TT
> can only succeed if the tablespace is empty.
    Some standby users may be actively using the tablespace via their
    <TT
CLASS="VARNAME"
>temp_tablespaces</TT
> parameter. If there are temporary files in the
    tablespace, all active queries are canceled to ensure that temporary
    files are removed, so the tablespace can be removed and WAL replay
    can continue.
   </P
><P
>    Running <TT
CLASS="COMMAND"
>DROP DATABASE</TT
> or <TT
CLASS="COMMAND"
>ALTER DATABASE ... SET
    TABLESPACE</TT
> on the primary
    will generate a WAL entry that will cause all users connected to that
    database on the standby to be forcibly disconnected. This action occurs
    immediately, whatever the setting of
    <TT
CLASS="VARNAME"
>max_standby_streaming_delay</TT
>. Note that
    <TT
CLASS="COMMAND"
>ALTER DATABASE ... RENAME</TT
> does not disconnect users, which
    in most cases will go unnoticed, though might in some cases cause a
    program confusion if it depends in some way upon database name.
   </P
><P
>    In normal (non-recovery) mode, if you issue <TT
CLASS="COMMAND"
>DROP USER</TT
> or <TT
CLASS="COMMAND"
>DROP ROLE</TT
>
    for a role with login capability while that user is still connected then
    nothing happens to the connected user - they remain connected. The user cannot
    reconnect however. This behavior applies in recovery also, so a
    <TT
CLASS="COMMAND"
>DROP USER</TT
> on the primary does not disconnect that user on the standby.
   </P
><P
>    The statistics collector is active during recovery. All scans, reads, blocks,
    index usage, etc., will be recorded normally on the standby. Replayed
    actions will not duplicate their effects on primary, so replaying an
    insert will not increment the Inserts column of pg_stat_user_tables.
    The stats file is deleted at the start of recovery, so stats from primary
    and standby will differ; this is considered a feature, not a bug.
   </P
><P
>    Autovacuum is not active during recovery.  It will start normally at the
    end of recovery.
   </P
><P
>    The background writer is active during recovery and will perform
    restartpoints (similar to checkpoints on the primary) and normal block
    cleaning activities. This can include updates of the hint bit
    information stored on the standby server.
    The <TT
CLASS="COMMAND"
>CHECKPOINT</TT
> command is accepted during recovery,
    though it performs a restartpoint rather than a new checkpoint.
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="HOT-STANDBY-PARAMETERS"
>25.5.4. Hot Standby Parameter Reference</A
></H2
><P
>    Various parameters have been mentioned above in
    <A
HREF="hot-standby.html#HOT-STANDBY-CONFLICT"
>Section 25.5.2</A
> and
    <A
HREF="hot-standby.html#HOT-STANDBY-ADMIN"
>Section 25.5.3</A
>.
   </P
><P
>    On the primary, parameters <A
HREF="runtime-config-wal.html#GUC-WAL-LEVEL"
>wal_level</A
> and
    <A
HREF="runtime-config-replication.html#GUC-VACUUM-DEFER-CLEANUP-AGE"
>vacuum_defer_cleanup_age</A
> can be used.
    <A
HREF="runtime-config-replication.html#GUC-MAX-STANDBY-ARCHIVE-DELAY"
>max_standby_archive_delay</A
> and
    <A
HREF="runtime-config-replication.html#GUC-MAX-STANDBY-STREAMING-DELAY"
>max_standby_streaming_delay</A
> have no effect if set on
    the primary.
   </P
><P
>    On the standby, parameters <A
HREF="runtime-config-replication.html#GUC-HOT-STANDBY"
>hot_standby</A
>,
    <A
HREF="runtime-config-replication.html#GUC-MAX-STANDBY-ARCHIVE-DELAY"
>max_standby_archive_delay</A
> and
    <A
HREF="runtime-config-replication.html#GUC-MAX-STANDBY-STREAMING-DELAY"
>max_standby_streaming_delay</A
> can be used.
    <A
HREF="runtime-config-replication.html#GUC-VACUUM-DEFER-CLEANUP-AGE"
>vacuum_defer_cleanup_age</A
> has no effect
    as long as the server remains in standby mode, though it will
    become relevant if the standby becomes primary.
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="HOT-STANDBY-CAVEATS"
>25.5.5. Caveats</A
></H2
><P
>    There are several limitations of Hot Standby.
    These can and probably will be fixed in future releases:

  <P
></P
></P><UL
><LI
><P
>     Operations on hash indexes are not presently WAL-logged, so
     replay will not update these indexes.
    </P
></LI
><LI
><P
>     Full knowledge of running transactions is required before snapshots
     can be taken. Transactions that use large numbers of subtransactions
     (currently greater than 64) will delay the start of read only
     connections until the completion of the longest running write transaction.
     If this situation occurs, explanatory messages will be sent to the server log.
    </P
></LI
><LI
><P
>     Valid starting points for standby queries are generated at each
     checkpoint on the master. If the standby is shut down while the master
     is in a shutdown state, it might not be possible to re-enter Hot Standby
     until the primary is started up, so that it generates further starting
     points in the WAL logs.  This situation isn't a problem in the most
     common situations where it might happen. Generally, if the primary is
     shut down and not available anymore, that's likely due to a serious
     failure that requires the standby being converted to operate as
     the new primary anyway.  And in situations where the primary is
     being intentionally taken down, coordinating to make sure the standby
     becomes the new primary smoothly is also standard procedure.
    </P
></LI
><LI
><P
>     At the end of recovery, <TT
CLASS="LITERAL"
>AccessExclusiveLocks</TT
> held by prepared transactions
     will require twice the normal number of lock table entries. If you plan
     on running either a large number of concurrent prepared transactions
     that normally take <TT
CLASS="LITERAL"
>AccessExclusiveLocks</TT
>, or you plan on having one
     large transaction that takes many <TT
CLASS="LITERAL"
>AccessExclusiveLocks</TT
>, you are
     advised to select a larger value of <TT
CLASS="VARNAME"
>max_locks_per_transaction</TT
>,
     perhaps as much as twice the value of the parameter on
     the primary server. You need not consider this at all if
     your setting of <TT
CLASS="VARNAME"
>max_prepared_transactions</TT
> is 0.
    </P
></LI
><LI
><P
>     The Serializable transaction isolation level is not yet available in hot
     standby.  (See <A
HREF="transaction-iso.html#XACT-SERIALIZABLE"
>Section 13.2.3</A
> and
     <A
HREF="applevel-consistency.html#SERIALIZABLE-CONSISTENCY"
>Section 13.4.1</A
> for details.)
     An attempt to set a transaction to the serializable isolation level in
     hot standby mode will generate an error.
    </P
></LI
></UL
><P>

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